This invention relates to an electroless gold method and also to electronic parts that are electrolessly gold plated by the method.
Hitherto, electroless nickel/immersion gold platings have been in frequent use for surface treatment for applications that require high reliability in mounting processes of printed circuit boards or electronic parts. In the immersion gold plating, gold is deposited by using a difference in redox potential between an underlying nickel layer and a plating bath, so that gold dissolves the nickel thereby corroding the nickel. In addition, diffusion of nickel over the gold film takes place, thereby lowering wire bondability. To avoid this, reduction gold plating is further performed on the electroless nickel/immersion gold plating films to make a thick gold film thereby suppressing the wire bondability from lowering, but with a problem of costs.
On the other hand, according to the recent lead-free promotion, there is a trend toward the use of Sn—Ag—Cu solder. However, a greater thermal load is needed upon solder bonding when compared with conventional tin-lead eutectic solders, with the attendant problem that bonding characteristics lower. To cope with this, there has been recently carried out a method of avoiding the above problem wherein a palladium film is sandwiched between the electroless nickel plating layer and the immersion gold plating layer according to electroless palladium plating.
It will be noted that mention is made, as related art technical literatures, of Japanese Patent Laid-open No. Hei 10-242205, Japanese Patent No. 3565302, Japanese Patent No. 3596335, Japanese Patent No. 3345529, Japanese Patent Laid-open No. 2004-332025, Japanese Patent Laid-open No. 2002-118134, Japanese Patent Laid-open No. 2006-339609 and Japanese Patent Laid-open No. Hei 8-269726.